A semiconductor device such as an IC (integrated circuit) generally has electronic circuit elements such as transistors, diodes and resistors fabricated integrally on a single body of semiconductor material. The various circuit elements are connected through conductive connectors to form a complete circuit which can contain millions of individual circuit elements. Integrated circuits are typically fabricated from semiconductor wafers in a process consisting of a sequence of processing steps. This process, usually referred to as wafer fabrication or wafer fab, includes such operations as oxidation, etch mask preparation, etching, material deposition, planarization and cleaning.
A summary of an aluminum gate PMOS (p-channel metal oxide semiconductor transistor) wafer fab process 40 is schematically shown in FIG. 1, illustrating major processing steps 41 through 73, as described in W. R. Runyan et al., Semiconductor Integrated Circuit Processing Technology, Addison-Wesley Publ. Comp. Inc., p.48, 1994. Each of these major processing steps typically include several sub steps. For example, a major processing step such as metallization to provide an aluminum layer by means of sputter deposition in a wafer fab chamber is disclosed in U.S. Pat. No. 5,108,570 (R.C. Wang, 1992). This sputter deposition process is schematically shown in sub steps 81 through 97 of process 80, see FIG. 2.
FIGS. 1 and 2 show sequential wafer fab processes. It is also known to utilize wafer fab sub systems which provide parallel processing steps. Such sub systems typically include one or more cluster tools. A cluster tool as defined herein includes a system of chambers and wafer handling equipment wherein wafers are processed in the cluster tool chambers without leaving a controlled cluster tool environment such as vacuum. An example of a cluster tool is disclosed in U.S. Pat. No. 5,236,868 (J. Nulman, 1993) which employs a vacuum apparatus having a central chamber and four processing chambers. A wafer handling robot in the central chamber has access to the interior of each the processing chambers in order to transfer wafers from the central chamber into each of the chambers while keeping the wafers in a vacuum environment. In one example, wafers in the '868 cluster are first transferred for processing to a cleaning chamber, then to a PVD (physical vapor deposition) chamber, followed by transfer to an annealing chamber and subsequently to a degassing chamber, thus utilizing a sequential process. It is also known to use cluster tools such as those disclosed in the '868 patent to process wafers in chambers which are used in parallel. For example, if a slow processing step is followed by a fast processing step, three chambers can be used in parallel for the slow process while the fourth chamber is used for the fast process.
It is well known to those of ordinary skill in the art that one or more processing parameters of a typical wafer fab process step need to be controlled within a relatively narrow range in order to obtain a product which has the desired characteristics. For example, U.S. Pat. No. 5,754,297 (J. Nulman, 1998) discloses a method and apparatus for monitoring a deposition rate during wafer fab metal film deposition such as sputtering. The '297 patent teaches that the metal deposition rate decreases with increasing age of the sputter target if the input sputter power level is maintained at a constant level. As a consequence, critical processing characteristics, such as the metal deposition rate, may vary from run to run for a given wafer fab processing chamber in ways that can affect the yield and quality of devices processed in that chamber. As disclosed in the '297 patent, the deposition system can be more readily maintained near desired levels when processing variables, such as the power input to the sputtering source, are adjusted in response to observed variations in the metal deposition processing characteristics. This requires in-situ measurement of processing characteristics, using for example a deposition rate monitor based on the optical attenuation of light passing through the deposition environment, thereby detecting the rate at which material is flowing from the deposition source to the deposition substrate, as described more fully in the '297 patent.
Advances in semiconductor materials, processing and test techniques have resulted in reducing the overall size of the IC circuit elements, while increasing their number on a single body. This requires a high degree of product and process control for each processing step and for combinations or sequences of processing steps. It is thus necessary to control impurities and particulate contamination in the processing materials such as process gases. Also, it is necessary to control processing parameters such as temperature, pressure, gas flow rates, processing time intervals and input sputter power, as illustrated in the '570 and '297 patents. As illustrated in FIGS. 1 and 2, a wafer fab includes a complex sequence of processing steps wherein the result of any particular processing step typically is highly dependent on one or more preceding processing steps. For example, if there is an error in the overlay or alignment of etch masks for interconnects in adjacent IC layers, the resulting interconnects are not in their proper design location. This can result in interconnects which are packed too closely, forming electrical short defects between these interconnects. It is also well known that two different processing problems can have a cumulative effect. For example, a misalignment of interconnect etch masks which is not extensive enough to result in an electrical short, can still contribute to causing an electrical short if the process is slightly out of specification for allowing (or not detecting) particulate contamination having a particle size which would not have caused an electrical short if the interconnect masks had been in good alignment.
Processing and/or materials defects such as described above generally cause a reduced wafer fab yield, wherein the yield is defined as the percentage of acceptable wafers that are produced in a particular fab. In-process tests and monitoring of processing parameters are utilized to determine whether a given in-process product or process problem or defect indicates that intervention in the process run is necessary, such as making a processing adjustment or aborting the run. Consequently, product and process control techniques are used extensively throughout a wafer fab. When possible, yield problems are traced back to specific product or processing problems or defects to ultimately improve the yield of the wafer fab. High yields are desirable for minimizing manufacturing costs for each processed wafer and to maximize the utilization of resources such as electrical power, chemicals and water, while minimizing scrap re-work or disposal.
It is known to use SPC (statistical process control) and SQC (statistical quality control) methods to determine suitable wafer fab control limits and to maintain the process within these limits, see for example R. Zorich, Handbook Of Quality Integrated Circuit Manufacturing, Academic Press Inc., pp. 464-498, 1991. SPC and SQC methodologies suitable for a wafer fab include the use of control charts, see for example R. Zorich at pp. 475-498. As is well known to those of ordinary skill in the art, a control chart is a graphical display of one or more selected process or product variables, such as chamber pressure, which are sampled over time. The target value of a particular variable and its upper and lower control limits are designated on the chart, using well known statistical sampling and computation methods. The process is deemed out of control when the observed value of the variable, or a statistically derived value such as the average of several observed values, is outside the previously determined control limits. Control limits are typically set at a multiple of the standard deviation of the mean of the target value, such as for example 2.sigma. or 3.sigma.. The target value is derived from a test run or a production run which meets such wafer fab design criteria as yield, process control and product quality. SPC and SQC are considered synonymous when used in the above context, see R. Zorich at p. 464.
Effective wafer inventory management is necessary for maintaining inventories of unprocessed or partly processed wafers at a minimum and thereby minimizing the unit cost of the semiconductor devices which are produced in the wafer fab. Minimizing inventories of wafers in process also has a wafer yield benefit because it is well known that the longer wafers are in the process, the lower their yield. Wafer inventory management typically uses scheduling techniques to maximize equipment capabilities in view of the demand for processed wafers, for example by scheduling parallel and series processing steps to avoid processing bottlenecks. Effective inventory control of a wafer fab also requires a low incidence of bottlenecks or interruptions due to unscheduled down times which can for example be caused by unscheduled maintenance, interruptions resulting from processing parameters which are outside their specified limits, unavailability of required materials such as a process gas, unavailability of necessary maintenance replacement parts, unavailability of a processing tool such as a chamber, or electrical power interruptions.
Many components or sub-systems of a wafer fab are automated in order to achieve a high degree of processing reliability and reproducibility and to maximize yields. Wafer fab tools such as chambers are typically controlled by a computer using a set of instructions which are generally known as a recipe for operating the process which is executed by the tool. However, it is recognized that a high degree of automation wherein various processes and metrologies are integrated, -is difficult to achieve due to the complexity and inter dependency of many of the wafer fab processes, see for example Peter van Zandt, Microchip Fabrication, 3.sup.rd ed., McGraw-Hill, pp. 472-478, 1997.
Wafer fabs require effective maintenance scheduling in order to maintain reliability of all components in the wafer fab. This generally results in having a costly spare parts inventory, thus adding to the IC production cost.
Electrical circuit breaker ratings for wafer fabs are generally much higher than the average power usage rate due to the need to absorb power surges in wafer fab equipment during processing. The high circuit breaker ratings require costly equipment in order to accommodate the power surges and peak power demands.
Accordingly, a need exists for methods and techniques which provide improved process control, quality, yield and cost reduction.